1. Field of the Invention
The present invention relates to an exposure technique for exposing a substrate with an exposure light beam through a liquid, a maintenance technique for an exposure apparatus using the exposure technique, and a technique for producing a device using the exposure technique.
2. Description of the Related Art
A microdevice (electronic device), which includes a semiconductor device, a liquid crystal display device, etc. is produced by the so-called photolithography technique wherein a pattern, which is formed on a mask such as a reticle, is transferred onto a substrate such as a wafer which is coated with a resist (photosensitive material). In order to transfer the pattern on the mask onto the substrate via a projection optical system in the photolithography step, those used are an exposure apparatus (so-called stepper) of the reduction projection type based on the step-and-repeat system and an exposure apparatus (so-called scanning stepper) of the reduction projection type based on the step-and-scan system.
As for the exposure apparatus of this type, it has been carried out to shorten the wavelength of the exposure light and increase the numerical aperture (NA) of the projection optical system (realize the large NA) in order to respond to such a request that the higher resolution (resolving power) is demanded year by year as the pattern becomes fine and minute accompanying with the realization of the high integration of the semiconductor device or the like. However, when the wavelength of the exposure light is shortened and NA is increased, then the resolution of the projection optical system is improved, while the depth of focus is consequently decreased and narrowed. Therefore, if such a situation is continued, then the depth of focus is too narrowed and it is feared that the focus margin may be insufficient during the exposure operation.
In view of the above, an exposure apparatus, which utilizes the liquid immersion method, has been developed. The liquid immersion method is such a method that the exposure wavelength is substantially shortened and the depth of focus is widened as compared with those obtained in the air (see, for example, International Publication No. 99/49504). In the liquid immersion method, the exposure is performed in such a state that the liquid immersion area is formed by filling a space between the lower surface of the projection optical system and a surface of the substrate with a liquid including water, organic solvents, etc. By doing so, the resolution can be improved and the depth of focus can be magnified about n times by utilizing the fact that the wavelength of the exposure light is 1/n-fold in the liquid as compared with the wavelength in the air (n represents the refractive index of the liquid, which is, for example, about 1.2 to 1.6).